The present invention relates to a light measuring device of the type having a dual-slope A/D converter wherein an electric charge of an amount which is commensurate with an amount or intensity of light measured by a photoelectric means is stored in one or more capacitors and is measured by measuring the time period during which the charge stored in the capacitor or capacitors is lowered to a predetermined level by a predetermined level of constant current, so that the measured time represents the measured amount or intensity of light. The measured time is usually converted to a digital signal such as, for example, the number of pulses counted during that time.
While a dual-slope A/D converter is advantageous in providing an accurate A/D conversion, conventional dual-slope A/D converters have had disadvantages in that they are slow in operation and cannot simultaneously convert a plurality of signals. Furthermore, when an input signal varies widely, e.g. from 0 to 2.sup.16, the time required for the conversion varies greatly.
The increase the speed of the dual-slope A/D conversion, some approaches have been made, such as the Triple-Ramp System, the Series-Integration System and the Quadraphasis System. These systems are designed to switch the rate of discharge, i.e. the level of the discharge current, per one clock period during the discharge process, thereby increasing the speed or rate of the conversion while maintaining its accuracy. In such systems, A/D conversion is effected for the full range with the minimum resolution maintained constant. However, such a full range conversion with a constant resolution is not always necessary, but it is sometimes the case that digital signals with only a particular number of bits are necessary while the minimum resolution may be shifted in accordance with the magnitude of the signal to be converted. For example, upon the conversion to sixteen bits of a digital signal, a signal of ten bits may be necessary while the minimum resolution may vary between 1, 2, 4, 8, 16, 32 and 64 bits. For such a case, the above noted systems operate with unnecessarily high accuracy.
Furthermore, in the above noted systems, the moment when the level of the discharge current is switched during the discharging operation must be synchronized with the clock pulse signal used for measuring the time period.
Furthermore, the time required to carry out the A/D conversion of data for a high intensity or amount of light takes a considerably longer time as compared with the time required for a low intensity or amount of light. Therefore, when sequentially carrying out a plurality of measuring operations according to a previously arranged program, it is difficult to control the timing relationship between the operations.
In order to overcome the above described disadvantages, there has been proposed an improved type of range changing system, e.g., disclosed in H. Ohtsuka et al.'s Japanese Patent Application which was laid-open to public inspection as (Tokkaisho) No. 53-63843, and published June 7, 1978, in which a plurality of capacitors and a switch coupled to each capacitor are provided, the capacitors being connected in parallel. When a current to be integrated is present, the switch coupled to the first capacitor is turned on to effect the integration of current only in the first capacitor. During the integration, the voltage across the first capacitor is measured, and when it reaches a predetermined level, the switch coupled to the second capacitor is also turned on so as to continue the integration in both the first and second capacitors. The turning on of the second switch is automatically carried out, and thus, it can be said that the range is automatically changed. In this manner, the switches are sequentially turned on so as to discretely increase the total capacitance, and when the integration is over, the number of switches which have been turned on is detected so as to obtain the total capacitance of the capacitors used for the integration, and, at the same time, the voltage across the capacitors is measured so as to determine the total charge stored in the capacitors.